The formation of molded products, typically hollow form products by rotational molding is widely practiced. Colored products have been formed by incorporating a pigment in the powdered plastic that is charged to the molds and the subsequent tumbling of the molds in a heated oven along two axes thoroughly incorporates the pigment throughout the molded product. This procedure, however, can only be used to produce a single colored product and decorative trim and/or printed matter cannot be imparted to the product by this procedure.
By far, the most prevalent plastic used for rotomolding is polyethylene, usually high density polyethylene. This material is extremely resistent to accepting printed matter or decoration, usually requiring some pretreatment such as flame oxidation and the like to render the surface receptive to colored paints, inks and the like. Other methods have been to apply decoration and printed matter to the molded surfaces in the form of adhesive decals and the like.
Although rotomolding is known and has been practiced for many years, no technique has yet been developed for imparting printed matter and decoration to the surface of rotomolded products during the rotational molding procedure. The rotational molding procedure briefly comprises charging the powdered plastic to a metal mold which is closed and supported on an arm for rotation about two major axes. The mold is placed inside an oven and heated therein to molding temperatures, typically from 500.degree. F. to about 800.degree. F. while it is rotated about two major axes, tumbling the powdered plastic against the heated interior mold surfaces where the powders adhere and collesce into an integral product. Methods for imprinting or decorating the surfaces of the molded product have, heretofore, been believed to be inapplicable to rotomolding because of the random mixing and tumbling of the polymer particles and pigment within the mold during the molding step. Consequently, no technique has heretofore been developed for achieving a molded pattern of printed characters or decoration in the surfaces of rotomolded products.